The need for low emissions fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production.
Fluidized and fixed bed processes have recently attracted renewed interest for increasing olefin production. For example, U.S. Pat. No. 4,830,728 discloses an FCC unit that is operated to maximize olefin production. The FCC unit has two separate risers into which a different feed stream is introduced. The operation of the risers is designed so that a suitable catalyst will act to convert a heavy gas oil in one riser and another suitable catalyst will act to crack a lighter olefin/naphtha feed in the other riser. Conditions within the heavy gas oil riser can be modified to maximize either gasoline or olefin production. The primary means of maximizing production of the desired product is by using a specified catalyst.
Often, conventional hydrocarbon conversion processes utilize crystalline zeolites as catalysts. Crystalline zeolites have been found to be effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce naphthas and olefins. Conventionally, crystalline zeolite is incorporated into a matrix in order to form a catalytic cracking catalyst.
It is also known that improved cracking results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 .ANG. is included with a crystalline zeolite having a pore size greater than 8 .ANG., either with or without a matrix, as set forth in U.S. Pat. No. 3,769,202. In this regard, a large pore size crystalline zeolite (pore size greater than 7 .ANG.) may be used in admixture with ZSM-5 type zeolite. The use of ZSM-5 in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934.
There remains a need, though, for improved catalysts that may be added to conventional (i.e., large pore) FCC catalyst, especially additive catalysts that result in improved activity and selectivity for light olefin formation, especially propylene formation.
Conventional silicoaluminophoshates such as SAPO-11 are catalytically active molecular sieves, especially at high silicon concentrations. However, such materials are prone to detrimental silicon distribution in the framework at high silicon concentration, above about 0.04 molar Si fraction in the framework, resulting in a diminished catalytic activity. Moreover, even materials with a favorable Si distribution in the framework may undergo an undesirable redistribution of framework Si during, for example, catalyst regeneration. There is therefore a need for improved silicoaluminophoshates that have increased catalytic activity at high silicon concentration, and improved activity maintenance characteristics.